Television programs, which can be transmitted as analog or digital transmissions via air, cable or a satellite link, include an audio signal and a video signal, both being modulated onto a carrier frequency. The television programs are arranged in respective frequency channels having a defined bandwidth and distance between each other within an allocated frequency range. For receiving a wanted television program, a tuner of a television receiver circuit has to be tuned to the respective channel, so that the television signal can be down-converted and demodulated.
A television receiver circuit for receiving of television programs comprises a mixer and a local oscillator, which is adjusted such, that for the reception of a wanted television program the respective program channel is down-converted by the mixer into a frequency range, which is within the passband range of a subsequent intermediate frequency (IF) filter. A television receiver of this kind is shown in FIG. 1: A television signal, received for example by means of an antenna, is amplified by an RF amplifier A, before it is coupled to a first input of a mixer 1a. To a second input of the mixer 1a a LO frequency signal of a local oscillator 1b is coupled, which is controlled by a channel selector 1c. The mixer 1a, local oscillator 1b and channel selector 1c constitute essential elements of a tuner circuit 1.
The channel selector 1c controls the local oscillator 1b such, that the television signal of a wanted television program is down-converted into a passband range of an intermediate frequency filter 2, for selecting one television signal out of a multitude of television signals within a frequency band.
The output of the IF filter 2 is coupled after amplification to a video detector 3, which provides a demodulated picture signal for a display on a video display monitor and a sound signal. The picture signal contains a horizontal synchronizing (sync) signal as known for controlling the horizontal scanning of the display monitor. The horizontal synchronizing signal is separated from the picture signal by means of a sync signal separator 4.
A television receiver of this kind comprises further an automatic fine tuning (AFT) circuit, because the television signal as transmitted by a respective television broadcaster might be shifted a little bit away from the allocated channel frequency by the broadcaster. The AFT circuit comprises an AFT discriminator 5, which is coupled to the output of the IF amplifier 2, and which provides an error voltage for the local oscillator 1b via a low pass filter 5a. The sync signal from the sync signal separator 4 can be used also for a first program search, by shifting the frequency of the local oscillator 1b in coarse steps through the frequency range of the allocated frequency band, and by detecting whether a sync signal is present or not for respective channel locations.
The horizontal sync signal can be used also for expanding the effective capture range of the AFT circuit, as described in U.S. Pat. No. 4,439,787. When using a television receiver in connection with a video tape recorder providing a video output signal by means of a RF modulator, which modulates the video signal of the tape recorder onto an unused television program channel for the tuner of the television receiver, it may happen that the AFT circuit of the television receiver captures the center frequency of a sound signal of an adjacent television program channel. This can be avoided by using the horizontal sync signal. When no horizontal sync signal is present, the local oscillator frequency is shifted accordingly.
The automatic fine tuning circuit provides a good solution for a reception of a television signal, but because of cost reasons it might be desirable to replace the automatic fine tuning circuit.
In U.S. Pat. No. 6,297,858, a method and apparatus for detecting video transmissions is described, which uses the presence of a synchronization signal to determine whether a video transmission is valid or not valid.
In U.S. Pat. No. 6,597,408 an automatic fine tuning method is described, in which the local oscillator frequency is shifted across the frequency range of a television channel, and when a sync signal is detected, the SNR and the respective frequency is stored. The frequency having the highest SNR is then selected as the tuning frequency for this channel.